Is there anything in the workbook that identifies which staff member is responsible for which customer? If so, one option would be to use a DATABASE field in a normal email mailmerge main document and a macro to drive the process. An outline of this approach can be found at:
Many to one email merge using tables - Microsoft Community Or, if your Excel workbook has a separate worksheet that with just a single instance of each of staff member's name, a DATABASE field in a normal ‘email’ mailmerge main document could be used without the need for a macro. An outline of this approach can be found at:
Mail Merge - To a Word Table on a Single Page - Microsoft Community